1. FIELD OF THE INVENTION:
The present invention relates to a process for dephosphorizing molten pig iron. More specifically, it relates to a process for lowering the phosphorus content with a small decarburization amount of 1.5 wt.% or less, by treating molten pig iron at less than 1,450.degree. C prior to charging it into a steel-making furnace.
2. DESCRIPTION OF THE PRIOR ART:
It has been known for a long time that phosphorus in steel is a harmful impurity since it causes temper brittleness and it lowers the product toughness. For this reason, dephosphorization of steel has been an important problem in steel production. It is no exaggeration to say that the progress of the steel-making process has been directly related to the progress of the dephosphorization process. However, the steel-making process has changed little since the invention of the LD converter after World War II which provides for a high dephosphorization rate. It is now the main technique in the steel-making process.
However, in recent years the kinds of steel requiring high toughness such as steel sheet for low temperature use and high tension steel have become more important, and these steels cannot contain phosphorus in amounts of more than 0.010 to 0.015 wt.%.
It is difficult to produce such low phosphorus steel by the normal single slag converter oxidation smelting technique, and thus a double slag converter oxidation smelting technique has been employed.
According to the condition of the raw materials, there are instances where pig iron containing phosphorus in amounts of far more than only 0.1 to 0.2 wt.% is used, and in such cases the double slag technique has to be employed if steel conforming to the severe phosphorus standard as mentioned above is to be produced, or even if ordinary steel containing not more than 0.035 wt.% of phosphorus is to be produced.
However, the double slag technique involves grave problems due to disadvantages such as the drop of converter productivity caused by long smelting time, the drop of the iron yield caused by intermediate slagging off, etc.